Method for building a bridge and bridge built according to said method

ABSTRACT

Method for building bridges and bridge built according to the method especially a girder bridge with one, two or more girders or longitudinal beams resting on supports at abutments or the like. The novelty lies therein that the girder or longitudinal beams have end portions extending beyond the supports at the abutments and that between such extending ends and the abutments are arranged anchoring devices which after being fixed to the ends of the extending ends of each girder exert a supporting moment to the portion of the girders between the abutments.

TECHNICAL FIELD OF THE INVENTION

This invention relates to the building of bridges and especially to thebuilding of girder bridges of the kind where one, two or morelongitudinal beams of steels rests against supports at their ends andsupports a roadway.

GROUND OF THE INVENTION

Conventional girder bridges have up until now been made as bridges witha main supporting structure of steel or aluminium with or withoutinteraction with the roadway whom the roadway as a rule is made ofconcrete.

Bridges are loaded by their inherent dead load and by movable load suchas traffic load, against its horizontal portions by earth pressure andbesides that from movements caused by temperature changes and whenconcrete is used in composite, also its creep and shrink.

Besides dimensioning the bridges with consideration to strength thedeformation of the construction because of traffic load must not exceedgiven values.

The quality of steel for the supporting girders has been improved sothat it has both a highter yield point and is more easily welded. Thestrength of the new steels qualities cannot be taken advantage of as theallowable deformations of the roadway normally has been used up longbefore the strength of the steel is fully utilized.

Steel girder bridges because of that will have at large spans highgirders with a low exploitation of the potential strength of the steeltogether with the fact that the volume of steel in the construction islarge.

Bridges over existing traffic roads such as roads or railways orwaterways normally demand bypass arrangements for existing traffic ortemporary closing thereof or speed reductions on the crossing trafficroad. Such restrictions of the traffic are very expensive not only withregard to economics because of increased transport work but also fromcompensations required by the holder of the earlier traffic rights. Forinstance restrictions in train traffic may result in demands forcompensation from the right of way holder to amounts comparable with thebuilding costs for the bridge.

Bridges over for instance railways or motorroads are normallyconstructed with one or more intermediary supports in order to avoid theneed of very heavy main girders as carriers. If such a bridge were to beconstructed with only one frame very heavy carring girders would benecessary which from an estetical point of view would be unwanted andresult in a construction so heavy that launching out or lifting out of aprefabricated bridge alternatively a partly prefabricated supportingstructure were unrealistic.

ASPECTS OF THE INVENTION

One aspect with the invention is to make it possible to build bridgesover traffic roads without disturbing the flow of traffic in anyessential degree. This means that the works with the bridge abutments inmost cases will have to be done without disturbing of the flow oftraffic on the lower traffic road and only exceptionally intermediarysupports for supporting the bridge girders will have to be done. Oneaspect is that a bridge over a standard clearance section for railwayfor fast trains or a bridge crossing a motor road is to be constructedwith a minimum influence on traffic, i.e. restriction of traffic duringa few hours preferably low traffic hours only.

SUMMARY OF THE INVENTION

The invention is based on the idea that the bridge is made as a framewith a predetermined support moment for dead weight over the frame legsindependently of the dimensions of the bridge. This support moment mayvary from 0 and up to the value which from the view of the function isthe most optimal one.

The predetermined support moment of dead weight is acheived bypositioning the entire ready super structure of the bridge or partsthereof including at least the supporting main beams or steal girdersfreely on the abutments for the bridge. The abutments are made withgreat stiffness as for example concrete plates. The abutments areprovided with supports for the main beams dose to the side of theabutments facing the bridge opening. On the opposite side of eachabutment at a distance from the supports of about 2.5 m in the lengthdirection of the bridge, there are in line with each girder positionarranged a cavity or recess extending downwardly from the level of theunderside of each girder. These cavities are each arranged to receive atleast one anchoring device attached to each end of the main girders andpreferably at each main girder extending and acting in the direction ofeach frame leg said anchoring device adapted to the dead weight and/orother predetermined load such as part of the dead weight, in order tocreate for loads over the predetermined one result in a supportingmoment at the main girder in which the predetermined load will result ina corresponding moment in the range of around 0. After having appliedthe predeterment load, for instance on lifting the main beams in theirposition resulting in a certain deflection thereof the anchorings areactivated which takes place by fixing them by means of concrete fillingup the cavites. The result of the load thus provided will be that thesupport moment of the main girders will be approximately 0 and the loadon the under soil over the frame legs will be approximately onlyvertical resulting in a load on the sub soil or sub ground over theframe leg in a vertical direction only. The result will be a relievingor deloading via the base slab or plate attached to the frame leg, ofthe unwanted horizontal forces biassing the soil stability of thesubground.

The statical function of the bridge wills thanks to the way of arrangingthe same, to be changed in such a way that the main girders will betightly clamped, so that the deflection caused by traffic will besmaller, in turn resulting in that the span may be increased withoutincrease of dimensions of the principal load carrying system, in thebest case up towards 50% compared with what has been regarded aspossible with known building methods and constructions. Motor-roads andbigger trafficroads thereby can be crossed without the use ofintermediary supports.

The load added before the securing of the anchoring devices by means ofconcrete casting, may be applied in different ways for instance byloading the principal load carrying system later on to carry the trafficload or part of the system with a temporary load which can be removedafter securing of the anchoring devices. A load may be chosen withdifferent distribution and with different signs in order to reach anoptimum result for the bridge. The load may also be equal with a deadweight of the superstructure added in this stage or part thereof.

According to an alternative method the anchoring devices secured to themain girders are fastened to the base plate of the abutments by means ofrelatively seen weak and adapted to considering the expected functionanchoring rods. This method is especially suitable in such cases thesuperstructure of the bridge is made as in situ cast concrete plate ormounted on the beams in the shape of prefabricated units. During thecasting in situ and the assembling of the bridge units, respectively,the beam ends extending beyond the abutments or supports will rise--asthe main portion of each beam between the supports is bent down--andthis results in a stretching of the anchorings. The anchoring rods areto be dimensioned in such a way in relation to the expected loads thatthe material of the rods as a result of the loading will be subject toplastic deformation.

This means that the part of the force exerted by the anchoring deviceswill be dependent on the area and quality of the anchoring materialselected and be essentially independent of the length of the anchoringrods or devices. If a concrete or other type of road way was placed onnon anchored girders an essential portion of the carrying ability of themain girders would be taken in use for supporting concrete or for theroadway only. By adding a force to the free ends of the girders whichforce may be varied in any desired way, the field moment of the maingirders may be deloaded to any desired extent which increases the spacefor traffic load.

The method according to the invention results in a maximum utilizationof the properties of high quality steel which also means that thebridges can be made so slender that maximum allowable deflection fortraffic load can be taken advantage of. Another essential advantage isthat the consumption of the steel material will stay close to a minimumand the construction as such compared with other structure will be verycheap.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in details in the following withreferences to the attached drawings, in which

FIGS. 1, 2 and 3 schematically illustrate in side view the building of abridge under utilizing a preferred method according to the invention,

FIGS. 4, 5 and 6 in a similar way illustrate the building of a bridgeaccording to embodified embodiment of the invention and

FIGS. 7, 8, 9 and 10 illustrate the procedure of arranging the supportsfor bridges erected according to the method for the invention and

FIG. 11 is a top view showing the support in the arrangement accordingto FIG. 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the drawings T indicates the position of a trafficroad which can be adouble track railway, motorway or the like and over this traffic route abridge is to be erected.

The bridge in its ready state includes a principal load carryingstructure 1 consisting of elongated girders, abutments 2, supporting theprincipal load carrying structure and a superstructure 3 which in turnis supported by the principal load carrying structure 1.

As girders or beams 4 including in the principal load carrying structure1 are used steel girders of high quality steel. The girders shown arepre bent.

The abutments 2 include a base slab or plate 5. With a limited extensionin the length direction of the bridge and on this base slab or plate theabutment proper rests. Each abutment includes a slab shaped centralportion 6 and at each end of this there are rearwardly, seen from theposition of the bridge, extending side walls 7 at the rear edge of eachof which a recess or cavity 8 is arranged. The rear surface of therecess may be provided with per see known pins or the like increasingthe shear resistance. Beam supports 10 for the girders 4 are arranged atthe upper edge of the central portion. In the illustrated embodimentwhich include two girders 4 a support 10 is arranged at either end ofsaid surface.

The principal load carrying structure includes in all cases two parallelgirders 4 with necessary transfers bindings or bracings.

FIG. 1 illustrates how the girders 4 are set on the supports 10. At theends of the girders 4 the so called anchoring devices 11 are arrangedand they include one or more rods with nobs or the like adhesionincreasing projections and the rods are by means of welding secured tothe ends of each girder 4. At the girders, which are of I-type in orderto provide stiffness between the flanges at the ends of the girder thereare preferably mounted, not shown, stiffening plates at each sides ofthe web. Similar stiffenings may also be arranged opposite the placeswhere each girder engage the supports 10. The anchoring rods 11 extenddownwardly into the recesses 8 at the abutments 2 essentially at rightangles to the longitudal direction of each girder.

On placing the girders 4 on their supports 10, the girders will changetheir shapes, bend downwardly and in the embodiment shown become morestraight. This results in that the anchoring rods 11 secured to the endsof the girders extending beyond or outside the abutments 2 will bedisplaced upwardly. After the girders having reached their rest positionthe anchoring rods 11 are to be secured by casting concrete into therecesses or cavities 8 preferably in connection with the casting of thewing walls 12 or mounting thereof if they are prefabricated.

The bridge deck may be pre mounted on the carrying structure especiallyif it, which is to be preferred, consists of so called aluminium planks.In the example shown the bridge deck 13 (FIG. 3) will be mounted inconnection with the erection of railings 14 and the fitting of otherthings. The permanent downward deflection is essentially fixed as theextending girder ends are anchored by means of the anchoring rods 11.

The bridge as a whole can after the anchoring of the ends of the girdersbe regarded as a bar rigidly secured at its ends and this implies thatthe downward deflection will be of another magnitude than arrangingsimilar girders to rest freely.

After the rigidly fixing at the supports of the anchoring rods attachedto the ends of the girders beyond the supports a new load case appears.The girders acting like rods with rigidly secured ends will have astrongly reduced tendency for downward deflection because of a load putthereon.

By the arrangement of the anchoring rods 11, the carrying structure andthe supports will co-operate like a unit in such a way that essentiallyonly vertical load will be transferred via the supports to the soiladjacent the supports. The structure does not act as a frame for thedead weight.

The method illustrated in FIGS. 4-6 starts with a pair of girders 4. Thegirders are along their upper side provided with pins or the likeincreasing the adhesion. Besides the anchoring devices 11 of a kindessentially similar with the one of the former examples there are at theends of the girders 4 also arranged so called temporary anchoringdevices 15, which are attached to the support 10, alternatively the baseplate 5 and extend upwardly towards each girder end where they areaffixed. As a casting of the bridge deck proceeds and the load on thegirders 4 results in a downward deflection, the anchorings 15 are moreand more put under tension. The dimensioning of the anchorings 15 is soadapted that the material of the rods constituting the anchorings 15starts to yield. Said rods thereby will create constant or rigidessentially non elastic anti load against the upward movement of the endof the girders caused by the downward deflection of the girder.

After finishing the casting of the bridge deck, the anchoring rods 11attached to the ends of the beams will be secured by concrete casting intheir recesses 8 in a way similar the one described with reference toFIGS. 1-3 and thereupon installation of wing walls, railings and surfacecoating takes place.

One essential advantage with this method is that, during the casting ofthe bridge deck, a supporting moment which up to a predetermined limitreduces the downward deflection of the bridge and essentially reducesthe field moment is added which contributes to the reduction ofmaterial.

In other cases to provide controlled supporting moments influencing themain support structure in same way as in FIGS. 4-6, anchorings devices15 may be secured to the abutments and stretched towards yield. This canbe done with jacks or rams or the like before securing the anchoringstowards the ends of the girders. Stretching of the anchoring devices 15secured to the ends of the girders may also be done by initially placingthe girders 4 on provisional supports and raising the girders by meansof jacks or the like until the material of the anchorings approaches ayield state and then putting in the permanent supports.

The method according to invention makes ft possible to prepare andarrange the principal load carrying structure, i.e. the girders 4according to FIGS. 1-3 and in the girders initially included in the moldaccording to FIGS. 4-6, at a distance from the intended position of thebridge. As soon as the supports have been arranged the entire bridge orthe girders with molds, respectively can be lifted to the intended sitealternatively be lanced out with no influence but for very marginal onthe traffic road below.

I claim:
 1. Method of building a bridge comprising the steps of:(a)providing an abutment on each side of an area to be bridged; (b)providing a beam support on each abutment; (c) placing at least one beamon and extending between the supports so that distal ends of the atleast one beam extend beyond the supports in a length direction of thebridge; (d) securing an anchoring device on each distal end of the atleast one beam; and (e) further securing each anchoring device to itsrespective one of the abutments in order to counteract upwarddisplacement of the distal ends caused by downward loads acting on theat least one beam between the supports and provide a supporting momentto the at least one beam.
 2. Method according to claim 1, furtherincluding the step of loading the at least one beam prior to the furthersecuring step.
 3. Method according to claim 1, wherein each abutment isprovided with a recess and its respective anchoring device is insertedtherein and secured by concrete casting during the further securingstep.
 4. Method according to claim 2, wherein dead weight of the atleast one beam is used as a load during the loading step.
 5. Methodaccording to claim 4, wherein, besides the dead weight of the at leastone beam, another determined load is used as a load during the loadingstep before the further securing step.
 6. Method according to claim 4,wherein, the further securing step takes place after the load has causeda downward deflection of the at least one beam.
 7. Method according toclaim 6, further including the step of further loading the at least onebeam, the ends of which have been secured during the further securingstep, with a predetermined load of a bridge deck resulting in a yieldingof material in the anchoring devices.
 8. Method according to claim 7,wherein the anchoring devices are secured to a base slab at eachabutment.
 9. Method according to claim 3, wherein the anchoring devicesare secured to adjoining upwardly extending portions of their respectiveabutments.
 10. Method according to claim 7, wherein after the step ofplacing at least one beam on and extending between the supports, afurther anchoring device is secured between each distal end of the atleast one beam and its respective one of the abutments and apredetermined load is applied to the at least one beam to cause ayielding of material in each further anchoring device.
 11. A bridgecomprising:(a) an abutment on each side of an area to be bridged; (b) abeam support on each abutment; (c) at least one beam on and extendingbetween the supports so that distal ends of the at least one beam extendbeyond the supports in a length direction of the bridge; and (d) ananchoring device secured on each distal end of the at least one beam andto its respective one of the abutments to counteract upward displacementof the distal ends caused by downward loads acting on the at least onebeam between the supports and provide a supporting moment to the atleast one beam.
 12. Bridge according to claim 11 further including afurther anchoring arranged and dimensioned in order to give way byyielding of material and exert a predetermined anchoring force uponapplication of a predetermined load.